When sending signals over transmission lines, it is often necessary to match the characteristic impedance of the line at the signal source in order to avoid reflections and ringing in the transmission line. Transmission line impedances in the range of 50 to 200 Ohms are commonly utilized. Additionally, transformers with different turn ratios are often needed, which can result in a wide range of reflected line impedances to be matched by the signal source, which signal source typically drives the transmission line with a line driver. The line driver can have a very low impedance associated therewith, thus requiring an external matching resistor, or it can incorporate therein an internal impedance that provides an output impedance that matches the transmission line impedance.
One approach to driving the transmission line with a line driver is to match the impedance of the line driver driving the line with either an ideally zero impedance amplifier in series with a resistor or an ideally infinite impedance amplifier in parallel with a resistor. This approach has a disadvantage of a substantial increase in the power consumption due to power dissipated by the added resistor. The alternative is to design an amplifier with a finite output impedance equal to the characteristic impedance of the transmission line. To be able to use such an amplifier with different transmission lines or transformers, its output impedance should be programmable.